The present invention relates to a magnetic recording medium. More specifically, the present invention relates to a magnetic recording medium installed in a variety of magnetic recording devices, such as an external memory of a computer. Furthermore, the present invention relates to a method for manufacturing such a magnetic recording medium.
With a rapidly increasing demand for a high-density and low-noise magnetic recording medium in recent years, a variety of compositions and structures of a magnetic layer, a variety of materials for a non-magnetic under-layer and a seed layer, and the like have been proposed. In particular, there has been proposed a magnetic layer that is generally called a granular magnetic layer, in which ferromagnetic grains are surrounded by a non-magnetic non-metallic substance such as an oxide or a nitride.
For example, Japanese Patent Laid-Open Publication No. 8-255342 discloses that a non-magnetic layer, a ferromagnetic layer and a non-magnetic layer are sequentially laminated on a non-magnetic substrate. A heating process is then carried out to form a granular recording layer, in which, in order to decrease noise, ferromagnetic grains are dispersed in the non-magnetic layer. The magnetic layer is made of a cobalt alloy or a cobalt-based alloy. The non-magnetic layer is made of metal, oxide, nitride, carbon or carbide. U.S. Pat. No. 5,679,473 discloses RF (Radio Frequency) sputtering using a CoNiPt target with an oxidem such as SiO2, added thereto, to form a granular recording layer, in which grains of magnetic substance surrounded by non-magnetic oxide are dispersed separately, thus achieving high Hc and low noise.
The granular magnetic layer with the above arrangement is considered to achieve the low noise characteristics since the physical separation of magnetic particles by a non-magnetic nonmetallic grain boundary phase reduces the magnetic interaction between magnetic particles. This controls the formation of a zigzag magnetic domain wall in a transitional region of a recording bit.
The noise of the recording medium is caused by the fluctuation of magnetization that occurs according to the size of magnetic particles constituting the medium and the magnetic particle-to-particle interaction. To maintain a high S/N in conformity with the increase in the recording density, it is necessary to keep the number of magnetic particles per bit cell at a predetermined value or more, i.e. it is necessary to refine the magnetic particles. When there is a great exchange interaction between the magnetic particles, refining the grain particles does not necessarily refine a unit of magnetic inversion in many cases. Therefore, it is necessary to inhibit the particle-to-particle exchange interaction in order to refine a unit of magnetic inversion expressed by magnetic moment of activation. Further, in refining the magnetic particles, the magnetic particles themselves require magnetic anisotropy energy being large to a certain extent in order to achieve the magnetic characteristics (high Hc/Mrt) required for high-resolution recording without coming into a super-paramagnetic state. The granular structure in which the magnetic particles with high magnetic anisotropy energy are dispersed in a non-magnetic matrix is intended to satisfy all of the above strict requirements for higher S/N.
When a conventional CoCr metal magnetic layer is formed at a high temperature, Cr is segregated from Co magnetic grains to be deposited on grain boundaries to thus reduce the magnetic interaction between magnetic particles. On the other hand, a granular magnetic layer is capable of accelerating the isolation of grains of magnetic substance relatively easily because a grain boundary phase is made of non-magnetic non-metallic substance to thus make it easier the segregation of Cr compared with the conventional magnetic layer. Particularly, the conventional CoCr metal magnetic layer requires the increase in the temperature of a substrate to 200xc2x0 C. or more during the formation thereof in order to sufficiently segregate Cr, whereas the granular magnetic layer enables segregation of its non-magnetic non-metallic substance even if it is formed without heating.
In the case of a magnetic recording medium having a granular magnetic layer, it is necessary to add a relatively large amount of Pt to a Co alloy in order to achieve desired magnetic characteristics and, more particularly, high magnetic coercive force Hc. In order to achieve Hc of about 2800 Oe, the granular magnetic layer requires expensive Pt of 16 atomic %, whereas a normal CoCr metallic magnetic layer requires Pt of only about 8 atomic %. In recent years, there has been an increasing demand for very high Hc of 3200 Oe or more with the increase in the magnetic recording density, and thus, the granular magnetic layer requiring a large amount of expensive Pt is disadvantageous because it increases the manufacturing cost. There has also been a demand for decreasing media noise with the increase in the recording density.
Further, the granular magnetic layer causes deterioration of the magnetic characteristics and the electromagnetic conversion characteristics at a small Br xcex4 (product of residual magnetic flux density and layer thickness) since a clear granular structure cannot be formed due to the unstable grain growth in a region with a small layer thickness (initial growth region). As the small layer thickness of the magnetic layer increases with the increase in the recording density in the future, it is important to develop the technique for preventing the deterioration of the media characteristics in the initial growth region in the granular magnetic layer.
It is an object of the present invention to provide a magnetic recording medium which overcomes the foregoing problems.
To solve the above described problems, the present invention provides a magnetic recording medium in which at least a non-magnetic under-layer, a magnetic layer, a protective layer and a liquid lubricant layer are sequentially laminated on a non-magnetic substrate. The magnetic layer comprises ferromagnetic grains and non-magnetic grain boundaries formed of metallic oxide or carbide surrounding the ferromagnetic grains. A non-magnetic intermediate layer comprised of grains of non-magnetic substance and non-magnetic grain boundaries formed of metallic oxide or carbide surrounding the grains of non-magnetic substance is provided between the non-magnetic under-layer and the magnetic layer.
In one preferred form of the present invention, the non-magnetic intermediate layer is formed of two or more laminated layers having identical or different compositions.
In another preferred form of the present invention, the grains of non-magnetic substance in the non-magnetic intermediate layer are CoCr alloys or CoCrPt alloys.
In yet another preferred form of the present invention, the non-magnetic under-layer is made of Cr or Cr alloy.
In yet another preferred form of the present invention, the non-magnetic substrate is crystallized glass, reinforced glass, or plastic.
The present invention also provides a method for manufacturing a magnetic recording medium in which at least a non-magnetic under-layer, a magnetic layer, a protective layer and a liquid lubricant layer are sequentially laminated on a non-magnetic substrate, the method comprising the steps of a) laminating a non-magnetic under-layer on the non-magnetic substrate; b) laminating, on the non-magnetic under-layer, a non-magnetic intermediate layer comprised of grains of non-magnetic substance and a non-magnetic grain boundaries formed of metallic oxide or carbide surrounding the grains of non-magnetic substance; c) laminating, on the non-magnetic intermediate layer, the magnetic layer comprised of ferromagnetic grains and non-magnetic grain boundaries formed of metallic oxide or carbide surrounding the ferromagnetic grains; d) laminating the protective layer on the magnetic layer; e) laminating the liquid lubricant layer on the protective layer; and f) wherein each step is carried out without heating the non-magnetic substrate in advance.
In one preferred form of the present invention, the step of laminating the non-magnetic intermediate layer comprises laminating two or more non-magnetic layers each comprised of grains of non-magnetic substance and non-magnetic grain boundaries formed of metallic oxide or carbide surrounding the grains of non-magnetic substance, the non-magnetic layers having identical or different compositions.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.